The present invention relates generally to the filling of storage tanks, and more particularly to a device that prevents over-pressurization of storage tanks while filling the tanks with cryogens from a transportable supply tank that uses a high flow and high pressure pump.
In the field of cryogenics, an operator connects a transport or supply tank on a tank truck to a feed, fill or distribution line for filling a stationary storage or receiver tank. A transport pump used to fill the storage tank, typically a centrifugal pump, is connected along the distribution line and is typically mounted on the truck itself. The cryogen exits the storage tank through a supply pipe extending from the storage tank to a facility for use or further distribution of the stored cryogen.
During filling of the storage tank through the fill line, a working pressure must be maintained in order to prevent a detrimental change in the pressure of the cryogen in the supply pipe. This requires that the pump maintain at least the working pressure as well as provide extra pressure to overcome the frictional pressure drop of the cryogenic fluid flowing to the storage tank. To ensure acceptable pressure in the tank, the operator must monitor the filling process to maintain a steady pressure for filling.
Since the centrifugal pumps have high flow and pressure capability, the pumps can easily exceed the allowable working pressure and the upper pressure limit of the storage tanks. Accidents may occur if the safety valves on the storage tank malfunctions and the filling process is not being monitored properly.
The Industrial Gases Council (IGC) established standards in xe2x80x9cPrevention of Excessive Pressure in Cryogenic Tanks During Filling,xe2x80x9d and the Compressed Gas Association (CGA) established a position statement PS-8-1997 entitled xe2x80x9cProtection of Cryogenic Storage Tanks from Overpressure During Operator-attended Refill.xe2x80x9d These standards were developed to recommend proper high flow filling procedures and requirements for the feed lines for the tanks.
In response, some gas producers use analysis of an extensive database to ensure the safety of tank truck fleets and storage tanks. The database contains specifications including pressure capacities and pressure test results for each component on each fill system. This type of data system either deems that fill lines and circuits are safe or adds restrictions to certain fill lines and circuits to make them safe. This requires enormous amounts of data management for pressure calculations such that smaller operations may not be able to afford this option. Further, the analysis must include both tank truck and storage tank data. Changes to either can undermine the data since it requires recalculating the pressure requirements for the modified systems. The database management system also limits the pump performance and increases cost delivery when a storage tank is rated in a way that requires the feed pump to be operated at less than full capacity. Finally, since the database systems are merely a preventative measure and no automatic shut-valve is added to these systems, an upset or overfill event potentially permits the contents of the supply tanks to be emptied and pooled on the ground.
Instead of such an expensive monitoring system, another known system uses a fill termination device (shut-off valve) to control the pressure to the storage tanks. The shut-off valve is an expensive, large and heavy 1xc2xdxe2x80x3 globe valve attached near the inlet to the storage tank and directly to the feedlines. A sensor plumbed to the top of the storage tank sends a signal to a motor on the globe valve automatically actuating it upon detecting a threshold pressure. Specifically, the globe valve has a spring-tripped release to close the valve and stop the flow that is triggered when the pressure in the storage tank is greater than the working pressure for that specific storage tank. This type of valve does not allow liquid to pool during an overfill since it does not have an opening to release the flow.
This conventional fill termination device, however, will completely shut off the flow when threshold pressures are reached in the storage tank without also immediately turning off the pump supplying the cryogen, which can dead-head the pump. Dead-heading occurs when cryogen flowing near its boiling temperature is stopped in the pump while the pump blades are still turning, which further warms the cryogen and converts it to a gas. Rotating the pump blades through the gas increases the RPMs to dangerous levels causing wear or damage to the pump.
Dead heading the pump can be avoided by providing a bypass loop that includes a return line leading back to the supply tank or supply vehicle in order to keep the flow moving until the pump can be turned off. This configuration, however, can be even more expensive since it requires a second line and more complicated valve system equipment to be placed on each tanker vehicle.
Lastly, other known automatic shut-off valves have complicated mechanisms for shutting-off the flow, usually utilizing springs, balls or other moving parts that are difficult to produce, wear quickly with use and require frequent maintenance.
Accordingly, it is a main object of the present invention to provide an improved device for over-pressure protection of storage tanks that inexpensively and effectively prevents damage to storage tanks and harm to operators when a high flow, high pressure pump is used to fill a storage tank.
More specifically, an object of the present invention is to provide an improved device for over-pressure protection of storage tanks that effectively shuts-off flow to a storage tank and releases pressure when a threshold pressure is reached in the tank without dead heading a pump supplying the flow.
An additional object of the present invention is to provide an improved device for over-pressure protection of storage tanks that automatically shuts-off the flow to a storage tank without the need for constant monitoring of the pressure during a filling process.
A further object of the present invention is to provide an improved device for over-pressure protection of storage tanks that effectively shuts-off the flow to the storage tank without dangerously pooling the flow when release of pressure is required.
Yet another object of the present invention is to provide an improved device for over-pressure protection of storage tanks that is inexpensive, easy to maintain and will not require frequent replacement due to wear.
These and other objects of the present invention are discussed or will be apparent from the detailed description of the invention.
In keeping with one aspect of the present invention, a device for over-pressure protection of a storage tank provides a plunger and shut-off mechanism that automatically shuts-off flow during over-pressure situations and automatically resets when the pressure drops back to normal operating levels. The shut-off mechanism is configured to maintain some flow to prevent dead heading of a pump providing the flow. Also, the plunger and shut-off mechanism are easy to assemble and maintain.
More specifically, a device for over-pressure protection of a tank has a main body defining a main passage with an inlet and an outlet. The main body also defines a release passage branched from the main passage. A shut-off mechanism is movably disposed within the main passage and defines an open position for allowing a substance to flow from the inlet to the outlet and a closed position for at least partially stopping the flow. Additionally, a plunger is movably disposed within the main body and connects to the shut-off mechanism. The plunger has a weight sufficient to maintain the shut-off mechanism in the open position when the plunger receives less pressure than a predetermined amount of pressure from the flow. The plunger also has a weight sufficient to move the shut-off mechanism into the closed position when the plunger receives pressure at or above the predetermined amount of pressure.
In keeping with another aspect of the present invention, the shut-off mechanism in the over-pressure protection device is configured to block most of the flow while simultaneously directing over-pressured flow coming back from the storage tank toward a release passage for relief of the pressure. In more detail, a device for over-pressure protection of a tank has a main body defining a main passage with an inlet and an outlet. The main body also defines a release passage branched from the main passage. A shut-off mechanism is movably disposed within the main passage and is movable from an open position that permits all flow from the inlet to reach the outlet. The shut-off mechanism also defines a closed position that blocks most, but not all, of the flow entering the inlet from reaching the outlet while simultaneously directing flow from the outlet to the release passage.